1. Field of the Invention
The present invention relates to a method of stopping the operation of a phosphoric acid fuel cell using phosphoric acid as an electrolyte. More particularly, the present invention relates to a method of stopping the operation of a phosphoric acid fuel cell which can prevent crystallization of electrolyte when its operation is stopped without using any heating means and which enables the replacement of an atmosphere inside a main body of the fuel cell by a non-combustible gas without difficulty.
2. Description of the Prior Art
As is well known, fuel cells include a stack constructed by a plurality of unit cells stacked one on another which are each composed of a matrix carrying an electrolyte, and a pair of electrodes, i.e., a fuel electrode and an oxidizer electrode, that sandwich the matrix between them, and generate electricity by supplying a fuel gas containing hydrogen and an oxidizer composed of air or oxygen to the stack. The fuel cells include various types: ones which are classified into a phosphoric acid fuel cell, an alkali fuel cell, and a molten carbonate type fuel cell depending on the electrolyte and working temperature used. For the sake of safety, it has hitherto been a common practice to conduct gas replacement operations in which gases retained in supply and discharge lines for supplying and discharging fuel gas, respectively, inclusive of the main body of a fuel cell itself, are replaced with an inert gas such as nitrogen gas at the time of starting or stopping (inclusive of emergency stopping) of the operation of fuel cells. More particularly, in the case of starting from the fuel cells being in a stop mode, generally air or oxygen remains in fuel lines inside the fuel cell and if a hydrogen-rich fuel gas was supplied to the fuel lines in that state there would be a danger that an explosive gas mixture could be formed and there could occur an explosion. On the contrary, when the operation of the fuel cell is to be stopped, if the fuel cell was left to stand in such a state that some fuel gas remains inside the main body of the fuel cell, the pressure of the fuel gas could decrease due to internal discharge or variation of temperature to induce penetration of air from outside the system to the side of fuel, resulting in that there would be a danger that an explosive gas mixture could be formed. Therefore, the gas replacement operation is carried out in order to obviate the danger.
Conventionally, power generation has been achieved by retaining a state in which the matrix layer in the phosphoric acid fuel cell contains or carries phosphoric acid as an electrolyte in a concentration of near 100% by weight. However, when the temperature of the main body of the fuel cell decreases to a level of no higher than about 40.degree. C. after the stopping, the phosphoric acid in a liquid state begins to deposit as crystals, i.e., a so-called crystallization of liquid phosphoric acid occurs. If the crystallization occurs, crystallized phosphoric acid will due to its thermal stress impair the electrodes or give rise to damages such as deterioration of the water-repellent property of the electric base materials. This problem is particularly serious because the fuel cells could often be exposed to a low temperature as low as several tens of degrees below zero depending on environmental conditions of the place where they are arranged. Accordingly, a process has hitherto been used in which heating means such as an electric heater is provided with the main body of the fuel cell or its cooling means in order to heat the main body of the fuel cell to a temperature no lower than the above-described crystallization temperature while the operation is stopped. However, this process is disadvantageous in that it is uneconomical because power is wasted while the operation is being stopped. Another disadvantage of the process is that the process requires a mobile power source for heating when the fuel cell is designed so as to serve as a power source installment for movement and as a result area for movement or travelling is limited.
On the other hand, in the conventional fuel cell installments, it is necessary to provide a storage tank such as pressure bomb for storing and controlling an inert gas besides the lines for fuel and oxidizer, and supply it from the storage tank to a gas reaction line of the fuel cell whenever the operation of the fuel cell is to be started or stopped. The conventional system is disadvantageous in that in addition to the management of fuel, troublesome, time-taking management is always necessary which includes, for example, monitoring the amount of gas remaining in the storage tank for the inert gas, securing stock gas inclusive of spare gas and purchasing or procuring the inert gas. Particularly in the case of mobile power source installments, provision of a large storage tank is required for storing the inert gas, which is disadvantageous in that the installments must necessarily be of a large scale.